ROME (Reuters) - Pope Benedict said on Monday he will resign on Feb 28 because he no longer has the strength to fulfill the duties of his office, becoming the first pontiff since the Middle Ages to take such a step.

The 85-year-old pope said he had noticed that his strength had deteriorated over recent months "to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me".

"For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint Peter," he said according to a statement from the Vatican.

A Vatican spokesman said the pontiff would step down from 1900 GMT on February 28, leaving the office vacant until a successor is chosen.

(Reporting by Steve Scherer; editing by Janet McBride)